1. Field of the Invention
The present invention relates to color processing for implementing color matching between different viewing conditions.
2. Description of the Related Art
FIG. 1 is a view showing the definition of a field of view of a human in a color appearance model.
The color appearance model is designed to correctly predict the color appearance of a human when a color patch having a viewing angle of 2° is given. In general, a CIE 1931 standard calorimetric observer is applicable within the viewing angle range from 1° to 4°, and this viewing angle range is distinguished to a stimulus 61 within 2° and a proximal field 62 within 4°. A field within the viewing angle range from 4° to 10° outside the proximal field 62 is called a background 63, and a field around the background 63 is called a surround field 64. Furthermore, a viewing field including all of the stimulus 61, proximal field 62, background 63, and surround field 64 is called an adapting field.
As a typical color appearance model, the CIECAM02 is known, and this model can set the following viewing condition parameters:
La: the absolute luminance [cd/m2] (normally, 20% of an absolute luminance Lw of a white point of the adapting field) of the adapting field,
XYZ: the relative XYZ values of a color patch,
XwYwZw: the relative XYZ values of the white point,
Yb: the relative luminance of the background, and
surround condition: Average Surround, Dim Surround, or Dark Surround.
Note that the surround condition is “Average” when the relative luminance in the surround field is 20% or more of the white point in the adapting field, “Dim” when it is less than 20%, and “Dark” when it is nearly 0%.
Since the color appearance model is derived from experimental results using color patches of solid colors, viewing condition parameters to be applied to an image including a plurality of mixed colors are not settled. Since neutral gray is 20% of the luminance of the white point, it is a common practice to set 20% of the absolute luminance Lw of the white point in the adapting field as the relative luminance Yb of the background.
When the color appearance model is applied to an image, it is a common practice to apply one set of viewing condition parameters to all pixels. However, a problem remains in that the visual effect of a solid color and background expressed on a rasterized image cannot be reflected on the color matching result by the application of one set of viewing condition parameters to all pixels.
In other words, it cannot be said that color matching with respect to the image resolution and viewing distance of the input and output sides (to be referred to as spatial color matching hereinafter) has been performed in a conventional system. To solve this type of problem, iCAM proposed by Fairchild, reference 1 (Japanese Patent Laid-Open No. 2002-042132), and reference 2 (Japanese Patent Laid-Open No. 2006-215756) propose spatial color matching with respect to the impact of surrounding pixels.
iCAM merely experimentally defines about ¼ of an input image size as a low-pass filter, and does not consider the viewing distances of the input and output sides or the image resolution of the output side. The technique of reference 1 defines a field corresponding to a field of view of 10° of the output side with respect to an input image and considers the image resolution and viewing distance of the output side, but it sets the image resolution and viewing distance of the input side to be the same values as those of the output side. The technique of reference 2 defines a field corresponding to a field of view of 10° of the input side with respect to an input image and considers the image resolution and viewing distance of the input side, but it does not consider the image resolution and viewing distance of the output side. The techniques of iCAM and references 1 and 2 do not perform spatial color matching that considers the color reproduction range of an output device.